1. Field of the Invention
This invention relates to a method for and an apparatus for sealing containers with a flange formed on the outer periphery of the open end, and it is applicable to food or like containers which are sealed by fusing a thermoplastic sheet or film to them.
2. Description of the Related Art
It is a recent trend of food packages to use thermoplastic resin containers in lieu of metal cans. To seal such thermoplastic resin containers, a heat seal method is used extensively. In this method, a sheet or film of a thermoplastic resin, such as polyolefine (hereinafter referred to as sheet material) is overlapped over a flange formed on the outer periphery of the open end of the container, and in this state it is pushed from above with a seal bar heated to a constant temperature to thereby fuse it to the container.
In this heat seal system, a certain time is required for the heating of the container and the sheet material at the time of the fusing. Therefore, a limitation is imposed on the number of containers that can be processed in unit time. Accordingly, various sealing apparatuses have heretofore been proposed to Permit faster sealing operation.
Among the sealing apparatuses for fast sealing is one, which is of stamping type with a multiple row arrangement of sealing bars for pressing and fusing sheet material. There is also a box motion type sealing apparatus, in which seal bar is moved in a box motion. Further, there is a high frequency sealing apparatus utilizing a high frequency oscillator. Further, there is a sealing apparatus of supersonic wave sealing type, in which a high frequency voltage is applied to sheet material for fusion thereof with heat generated as a result of dielectric loss of the sheet material. Furthermore, there is a package container sealer, in which a sheet material is fused to a container with heated heat roller (as disclosed in Japanese Patent Laid-Open Publication NO. S61-103).
However, the prior art stamping type sealing apparatus using multiple rows of sealing bars, has disadvantages that it leads to increased size and cost.
The sealing apparatus of supersonic frequency sealing type is also expensive and unsuited to general purpose sealing. The high frequency sealing can not be applied to low dielectric loss sheets, i.e., thermoplastic resin sheets with no polarity. That is, limitation is imposed on the kind of sheet material that can be used.
The method of sealing package containers as disclosed in Japanese Laid-Open Publication No. S61-103, is readily conceivable by a person having ordinary knowledge in the art in that sheet material is fused to container by using heat roller. This method is thought to have an advantage in that it is applicable to different containers having various shapes. However, it has many basic problems concerning the stability of the seal, that is, wrinkles in the sealed portion, lack of uniformity of the seal, insufficient mechanical strength of the seal, rate of sealing, warping and other deformation of flange, etc.
The present invention is intended in the light of the above circumstances, and its object is to provide a method for and an apparatus of sealing containers, which permit faster and more inexpensive sealing of containers than in the prior art methods by fusing a thermoplastic resin sheet on a flange of the container and are applicable to different containers having various shapes.